If I boil diesel fuel in a reduced atmosphere, then pass it through a condenser, will it make diesel again?

Diesel fuel ,just like gasoline is a blended product .The diesel in other words is not elemental in composition..If you heat diesel to a point the lighter parts will "perk" off fastest. In a condensate tube its true they could recombine with their original counterparts to make a whole however the bonds they form will change. To get diesel again one would have to re-heat the recombinant to the correct temperature under the correct pressure to break down the newly constructed bonds to

re-refine the diesel.. I like the idea you have though of precipitating lights and mixing them with Bio-D .the cetane value would be raised i'm sure..During the transesterfication process if the fatty acids Aren't,t removed and you choose the wrong part of the distillate you might be putting paraffin in with the FA's. Which would cause instant injector clog. Wish you the best of luck though..From the E..

It's unlikely you'll thermally decompose regular diesel by vacuum distillation. If you collect all the fractions, then combine, you will end up w/ pretty much the same original product. However, it's likely there are some additives which will not boil off, meaning the recovered diesel will not be exactly the same as the original.

As a note of caution, try not to hurt yourself. The distillation of hydrocarbons is dangerous to the unskilled, and the scale required to produce any meaningful amount of fuel will make it even more so. Also, vacuum distillation means you need pumps, pressure rated tanks and condensers, and a strong cooling source to capture the vapors. An appropriate ventilation system is needed, and all electrical connections and system should be intrinsically safe and explosion proof.

i don't see why not ,but your only ending up back where you started,watch your pressures it might blow.